Radiation is widely used in interrogation and irradiation of objects, including people. Examples of interrogation include medical imaging, cargo imaging, industrial tomography, and non-destructive testing (NDT) of objects. Examples of irradiation include food irradiation and radiation oncology. Accelerated charged particles, such as protons, are also used in radiation oncology.
Radio-frequency (“RF”) accelerators are widely used to accelerate charged participles and to produce radiation beams, such as X-rays. RF accelerator based radiation sources may operate in a pulsed mode, in which charged particles are accelerated in short pulses a few microsecond long, for example, separated by dormant periods. Some applications require a “steady state” radiation beam, in which each pulse of radiation is expected to be the same. Other applications, such as cargo imaging, may use interlaced multiple energy radiation beams, as described, for example, in U.S. Patent Publication No. 2010/0038563A1 (“the '563 Publication”), which was filed on Aug. 12, 2008, is assigned to the assignee of the present invention, and is incorporated by reference herein.
FIG. 1 is a block diagram of major components of an example of an RF accelerator system 10 configured to generate radiation. The system 10 comprises an accelerator (also called beam center line (“BCL”)) 12. An RF source 14, which may be a magnetron or a klystron, provides RF power to the accelerator 12, through an RF network 16. The RF network 16 ensures that the RF source 14 is properly coupled with the accelerator 12, and isolates the RF source from reflected RF power and the frequency pulling effect caused by the accelerator. The RF network typically includes a circulator and an RF load (not shown). A charged particle source 18 injects charged particles into resonant cavities (not shown) of the accelerator 12, for acceleration. A target 20, such as tungsten, is positioned for impact by the accelerated charged particles, to generate radiation by the Bremsstrahlung effect, as is known in the art. To generate X-ray radiation, the charged particle source may include a diode or triode type electron gun, for example.
The RF source 14 is maintained in a “ready to generate” RF condition by a filament heater (not shown). The external surface of the RF source is usually temperature controlled. The charged particle source 18 also includes a filament heater (not shown) so that the particle source is ready to inject particles when requested.
An electric power supply 22 provides electric power to the RF source 14 and the charged particle source 18. The electric power supply is controlled by a controller 24, such as a programmable logic controller, a microprocessor, or a computer. An automatic frequency controller (“AFC”) 26 is provided to match the resonance frequency of the accelerator 12 with the frequency of the RF source 14, as described in the '563 Publication, identified above.
When a beam-on command is provided to the controller 24 by an operator, for example, the controller 24 turns on the electric power supply 22 to provide electric power to the RF source 14 and to the charged particle source 18. The electric power may be provided in the form of pulses of a few microseconds each, at a rate of up to a few hundred pulses per second, for example. The RF source 14 generates standing or travelling electromagnetic waves in the resonant cavities of the accelerator, which bunch and accelerate charged particles injected by the charged particle source 18. In this example, accelerated charged particles are directed toward the target 20. Impact of the accelerated charged particles on the target 20 causes generation of radiation by the Bremsstrahlung effect, as mentioned above at a corresponding radiation pulse length and rate. The electric power supply 22 is turned off when radiation is no longer desired. A beam-off command may be received from an operator or the controller may be programmed to end beam generation after a predetermined period of time. A beam run may last for seconds, minutes, or hours between a beam-on command and a beam-off command, for example. When radiation is desired again, the electric power supply is turned on again. Accelerated charged particles may also be used directly, in which case the target 20 is not necessary.
Typically, the RF pulses substantially coincide with particle injection in time. The RF generation and particle injection can also be controlled to only partially overlap in time, to control radiation output, as described in U.S. Patent Publication No. 2010/0177873, assigned to the assignee of the present invention and incorporated by reference herein.
The stability of a generated radiation beam may vary from the beginning to the end of the radiation beam. See, for example, Chen, Gongyin, et. al., “Dual-energy X-ray radiography for automatic high-Z material detection,” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms Vol. 261, Issues 1-2, August 2007, pp. 356-359. FIG. 2 is a graph of normalized radiation dose versus time for a continuous radiation beam 2a generated for over 300 seconds by a Varian M9 Linatron®, available from Varian Medical Systems, Inc, Palo Alto, Calif., based on actual test results. The continuous radiation beam 2a in this example comprises radiation pulses generated at a rate of several hundred pulses per second. Each pulse may last a few microseconds. These microsecond pulses are not indicated. In this example, the dose rate drops about 10% from a peak dose 2b at the very beginning of the radiation beam to a more steady dose rate after about 150 seconds. The energy of the radiation beam may vary, as well.
FIG. 3 is a graph of normalized radiation dose rate versus time for a plurality of shorter radiation beams 3a-3h, by the same Varian Linatron® during a cycle beam run. In this example, each radiation beam 3a-3h extends for a 10-second beam-on time period separated by a 5-second beam-off time period. In each 10-second beam-on period, the dose rate drops about 6% from an initial peak at the start of each beam. As above, each radiation beam 3a-3h comprises microsecond pulses of radiation microseconds long, generated at a rate of several hundred pulses per second. The energy of each radiation beam may vary, as well. Other commercially available linear accelerators may show instabilities similar to those shown in FIGS. 2 and 3.